In recent years, a coherent optical receiving system is spotlighted for realizing high-speed, large-capacity, and long-haul Wavelength Division Multiplexing (WDM) optical transmission systems. The coherent optical receiving system performs waveform equalization processing such as chromatic dispersion compensation by the digital signal processing after performing coherent reception of a light signal which transmitted in an optical fiber and converting into an electric signal. In other words, the coherent optical receiving system can compensate waveform distortion of a signal caused by chromatic dispersion or the like, which becomes a major problem in case of long-haul transmission in the optical fiber, with a high degree of accuracy using an electric equalizing filter. For this reason, the coherent optical receiving system does not require optical components for chromatic dispersion compensation such as DCF (Dispersion Compensating Fiber) and TDC (Tunable Dispersion Compensator). The point that such expensive optical components are not needed and a substantial cost reduction can be achieved becomes one of major advantages of the coherent optical receiving system.
An example of an optical receiver to which such coherent optical receiving system has been applied is disclosed in Japanese Patent Application Laid-Open Publication No. 2010-178222. The Japanese Patent Application Laid-Open Publication No. 2010-178222 discloses a distortion compensation apparatus which can reduce a hardware size of the distortion compensation apparatus and an optical receiver which includes that. This related distortion compensation apparatus includes a plurality of regulated amount compensation units in which a compensation amount is fixed and a variable compensation unit in which a compensation amount is variable, and compensates waveform distortion of a received light signal by the digital signal processing. The regulated amount compensation unit is configured so that it can change the combination of compensating operation by on/off switching. And, depending on the waveform distortion which arises in an optical transmission path, a controller first compensates the waveform distortion by the fixed compensation amount which can be compensated by a combination of the regulated amount compensation units, and then variably controls the compensation amount of the variable compensation unit based on the remaining waveform distortion.
Japanese Patent Application Laid-Open Publication No. 2011-009956 discloses the technology which solves the degradation of communication quality due to a frequency drift of a local oscillation light in the digital coherent receiving. If a frequency of the local oscillation light fluctuates, a light signal cannot be demodulated in digital domain with high accuracy. As a result, the communication quality degrades. According to the Japanese Patent Application Laid-Open Publication No. 2011-009956, a waveform distortion compensating circuit compensates waveform distortion of a signal converted into a digital signal, and a phase detecting unit detects a phase of the signal for which the waveform distortion has been compensated. And, a phase compensator compensates the phase of the signal for which the waveform distortion has been compensated by the waveform distortion compensating circuit based on the phase detected by the phase detecting unit, and demodulates the signal for which the phase has been compensated.
Japanese Patent Application Laid-Open Publication No. 2011-015013 discloses the technology which appropriately performs the dispersion compensation in a digital coherent optical receiver. In the technology disclosed in the Japanese Patent Application Laid-Open Publication No. 2011-015013, a first compensating circuit compensates the waveform distortion corresponding to chromatic dispersion by the digital signal processing. And, a chromatic dispersion compensation control circuit controls the compensation amount of the chromatic dispersion in the first compensating circuit based on a detection output value of a phase shift between a sampling signal in the digital conversion and a modulation frequency of a received light signal.